Because of lack of fuel for combustion processes, alternatives are being sought for electrically driven heat pump systems or heat driven heat pump systems using combustion processes to supply the necessary heat to drive the system. One alternative that has been suggested is to use solar energy to supply the necessary heat to drive a heat driven heat pump system rather than a combustion process. Two general types of heat driven, heat pump systems are available. The first type is an absorption system which uses heat to boil a refrigerant out of a carrier liquid in a boiler generator, passes the refrigerant through a condenser and an evaporator, and then recombines the refrigerant with the carrier liquid for recycling in an absorber. The second type is a dual loop system that has a power loop in which the power loop working fluid is heated and used to power an expansion-compression device. The heat pump loop of such systems is connected to the compression side of the expansion-compression device and operates on the vapor compression cycle.
With an absorption system, the minimum temperature required to operate such a system is relatively high. Presently available solar energy collection systems, on the other hand, are able to obtain this relatively high operating temperature required for an absorption system for only a short period of time during a 24 hour period under the best of conditions and in many instances not at all. This has required the use of a large collector associated with a thermal storage system to collect and store the high temperature heat energy when available for later use or a combustion process to supplement the heat obtained from solar energy for most of the required operating time of the absorption system thus making it uneconomical to use solar energy to drive an absorption system especially when the initial installation cost is considered.
One heat driven dual loop system that has been suggested uses a linear motion free-piston expansion-compression device such as that disclosed in U.S. Pat. Nos. 2,637,981 and 3,861,166. These free-pisotn expansion-compression devices have been able to operate effectively and efficiently only within very limited temperature ranges of heat input and in order to obtain reasonable efficiencies have also required relatively high minimum temperatures to drive the system. Because the heat output capability from presently available solar energy collection systems alwyas varies widely over a 24 hour period and also because these solar energy collection systems are able to collect heat at the required relatively high operating temperatures required for the dual loop system for only a short period of time during a 24 hour period under the best of conditions, it has been necessary to use a large collector associated with a thermal storage system to collect and store the high temperature heat energy when available for later use or a combustion process to supplement the heat obtained from solar energy for most of the required operating time of the system. Thus, like the absorption system, solar energy has been unable to economically drive a dual loop heat pump system with an expansion-compression device.